A modern direct drive wind turbine comprises a permanent magnet generator (PMG) in which the excitation field is provided by permanent magnets instead of a coil. Permanent magnet generators are widely used in wind turbines due to its high efficiency and low weight.
The document “WP131001EN-Generator circuit breakers have special requirements for generator protection” shows an example of circuit breakers which can be used in wind turbine generators.
Document EP 2 605 390 A1 shows a frequency converter which is able to convert the frequency of the output voltage of a wind turbine generator to the rather fixed frequency of a grid. The three-phase output voltage of the generator is being rectified to a DC signal, i.e. converted from AC to DC. The DC signal is then been converted into an output AC voltage signal adapted with a frequency which is adapted to the connected grid.
In modern direct drive permanent magnet wind turbine generators, Nedoymium Iron Boron (NdFeB) magnets are mostly used. The remanent flux density Br of a Nedoymium Iron Boron (NdFeB) is influenced by the ambient temperature. The temperature coefficient a of the remanent flux density Br (also called magnetic output), i.e. how Br varies with temperature, for a NdFeB magnet is typically −0.12%/° C. from ambient temperature, but a range of −0.08%/° C. to −0.12%/° C. is possible depending on the Neodymium content of the magnet.
There are several effects due to elevated temperatures. A reversible loss occurs when the magnetic output/remanent flux density Br falls with rising temperatures but returns as it cools down. As example, a 20° C. rise above ambient temperature with a magnet's typical temperature coefficient of a=−0.12%/° C. causes a drop in magnetic output of around 20° C.×0.12%/° C.=2.4%, which recovers when the temperature returns down to ambient temperature. An irreversible and just partly recoverable loss occurs when the magnetic output falls with rising temperatures but does not fully return when the magnet cools down.
In wind turbine generators, the generator's magnetic flux is proportional to the remanent flux density Br of the permanent magnets which are able to produce the magnetic flux or the magnetically output. Therefore, a change of the current remanent flux density of the permanent magnets, e.g. due to temperature, or ageing, will result in a respective change in the generator's magnetic flux.
The document “counter electromotive force” (Wikipedia) explains the meaning and the function of the back-electromotive force (back-EMF) of a motor.
The document “Magnets Faults Characterization for Permanent Magnet Synchronous Motors” (Dominico Casadei et al.) shows a method to detect partial demagnetization of permanent magnets in motor applications by measuring harmonic frequencies of the back-EMF voltage.